The present invention relates to medical ultrasound imaging, and in particular to imaging methods and systems that provide improved imaging of directional targets.
Objects of interest for medical ultrasonic imaging include resolvable targets with strong directionality such as membranes between different types of soft tissue, ducts, tendons, muscle fibers, and interventional devices such as biopsy needles. In general, objects with substantially planar or linear acoustic impedance boundaries within the round-trip sample volume of an imaging system behave as directional acoustic targets. They do not scatter incident sound waves isotropically, but reflect them back anisotropically such that the reflection angle with the surface/line normal is equal to the incidence (insonification) angle (Snell""s Law). For this reason, such directional targets locally have a narrow spatial or lateral bandwidth. If the imaging system spatial impulse response has a narrow bandwidth, i.e. the range of incidence angles is narrow and the receiver is responsive to substantially the same range of angles, directional targets will often not be detected when the incidence angle is substantially different from zero. On the other hand, if the system spatial impulse response has a wide bandwidth, i.e. the insonification subtends a wide range of angles and the receiver is responsive to echoes from a wide range of angles, then the signal-to-noise ratio (SNR) of the narrow bandwidth signals (e.g., reflections from a directional target) are compromised. In either case the detectability of directional targets may be significantly reduced.
One prior-art technique that improves the detectability of directional targets is spatial compounding, where the target is imaged multiple times from substantially different angles and the images are then combined after amplitude detection (Jago U.S. Pat. No. 6,126,599 and Schmiesing U.S. Pat. No. 6,135,956). With the conventional spatial compounding technique, however, the temporal resolution is sacrificed because of multiple firings needed for each frame of compounded image (Entrekin U.S. Pat. No. 6,126,598 and Robinson U.S. Pat. No. 6,210,328). The temporal resolution loss can be unacceptably high for applications that require high temporal bandwidth, or for applications that inherently have low frame rates, e.g., 4-D imaging. The conventional spatial compounding technique also suffers from motion artifacts if the transducer or object is in motion during the acquisition of component image frames (Jago U.S. Pat. No. 6,117,081). Images with conventional spatial compounding can also exhibit seam artifacts at the borders of component image frames (Jago U.S. Pat. No. 6,224,552). Thus, a need presently exists for an improved method for imaging directional targets that has a reduced adverse effect on the frame rate, reduced motion artifact and reduced discontinuities in the compound frame.
The methods and systems described below improve the contrast resolution of medical ultrasound images, particularly when directional targets of the type described above are imaged. The disclosed systems compound multiple images that are generated using only a single firing per ultrasound line or in some cases two firings per ultrasound line. Speckle variance is also reduced as a natural result of spatial compounding, and this further improves the detectability of soft-tissue lesions.
One system described below uses a bank of anisotropic band-pass filters prior to amplitude detection to create multiple component images. The other system creates multiple receive beams from a single weakly diverging, planar, or weakly focused transmit beam by using partially overlapping receive sub-apertures. Both the filtered images of the first system and the sub-aperture receive beams of the second system are selectively sensitive to directional targets oriented in respective ranges of angular positions. When the filtered component images (in the first system) or the sub-aperture receive beams (in the second system) are combined after detection, the desired improved imaging of directional targets is obtained. The first system preserves the frame rate and the second system allows spatial compounding with improved frame-rates. Therefore these systems address the temporal resolution loss issues of conventional spatial compounding and consequently provide reduced motion artifact.
The foregoing paragraphs have been provided by way of general introduction, and they should not be used to narrow the scope of the following claims.